1. Field of the Invention
This invention relates to a phosphor coated with pigment particles (hereinafter referred to as "pigment coated phosphor"), and more particularly to a pigment coated phosphor employed in a high contrast color television cathode ray tube.
2. Description of the Prior Art
As is well known in the art, pigment particles adhering to the surfaces of phosphor particles such as blue emitting phosphor particles, green emitting phosphor particles and red emitting phosphor particles employed in a color television cathode ray tube markedly enhance the contrast of the image formed on the cathode ray tube. This is because a part of the visible region of the emission spectra inherent in the phosphor is cut due to the filter effect of the pigment particles adhering thereto, which results in a clearer emission color, and further because the pigment particles can absorb a part of the external light to reduce the reflection of light thereby. This is disclosed, for instance in U.S. Pat. No. 3,886,394.
It is required for the pigment coated phosphor employed in a high contrast color television cathode ray tube mentioned above that the reflectance be low and the luminance be sufficiently high. That is, in order to lower the reflectance of the external light by the pigment coated phosphor, it is necessary to increase the amount of the pigment particles adhering to the surface of the phosphor and obtain a higher surface coating rate. In such a pigment coated phosphor having a high surface coating rate, the luminance thereof is inevitably lowered. It is required that the lowering of the luminance be controlled to be as small as possible. In other words, for a given specific reflectance, the luminance is desired to be as high as possible.
Heretofore, chromium oxide (Cr.sub.2 O.sub.3) have been recommended as green pigment particles for a green pigment coated green emitting phosphor employed in a high contrast color television cathode ray tube. The pigment coated phosphor using the chromium oxide pigment particles satisfies the aforesaid requirement to some extent, but not sufficiently. Therefore, it has not been put into practical use. That is, at present, blue and red pigment coated phosphors are used respectively as the blue and red emitting phosphors of a high contrast color television cathode ray tube, but a green pigment coated phosphor is not used as the green emitting phosphor thereof.
Under the aforesaid circumstances, there is demanded a green pigment coated green emitting phosphor available for practical use whose luminance is higher, when comparing phosphors of the same specific reflectance, than that of the conventional chromium oxide pigment coated green emitting phosphor.